Conventionally, as is widely known, bag filters have been used as filters for collecting dust exhausted from garbage incinerators, coal boilers, metal blast furnaces, and the like. The filter material of a bag filter requires heat resistance since the exhaust gas temperature is at a high temperature of 140° C. to 250° C., and also requires acid resistance and resistance to hydrolysis since acidic gases, such as SOx and NOx, and moisture are contained in the exhaust gas. Therefore, various types of heat resistant fibers such as polyphenylenesulfide (referred to as “PPS” below) fiber, meta-aramid fiber, polyimide fiber, fluorofiber, glass fiber, or the like have been used as the filter material, and non-woven fabrics, in particular, have been used.
Furthermore, in recent years, there has been a desire to filter as much exhaust gas as possible at a high collection efficiency, i.e., within a limited space, within this temperature range, and it is necessary to establish a large filtering surface area within a narrow space. Therefore, as described in the publication of Japanese Unexamined Patent Application, First Publication No. Hei 11-158776, a filter has been developed in which a synthetic resin is impregnated in a PPS non-woven fabric and a pleating process is carried out on the filter material. Furthermore, as the resin for impregnation, the publication of Japanese Unexamined Patent Application, First Publication No. 2001-192953, for example, discloses that if a melamine resin or phenol-formaldehyde resin is used as the resin for impregnation forming, then the heat resistance is very good, softening is difficult even at high temperatures, and the pleat shape retentivity at high temperatures during impregnation of the base material is good.
However, in the fiber base material in which a phenolic resin having a pH of 4 to 8 is impregnated into a non-woven fiber comprising the PPS fiber proposed in the publication of Japanese Unexamined Patent Application, First Publication No. 2001-192953 described above, when used for a long period of time at high temperatures, there were problems such as an abnormal degradation of strength, a large decrease in the tensile strength and folding endurance, and failure of the filter material during use over a long period of time. As an explanation for the cause of the above-mentioned degradation of strength, it is presumed that the degradation of strength occurred as a result of the multiplied breakdown effect due to the breakdown of the phenol resin resulting from an acidic gas such as SOx or NOx contained in the exhaust gas, or the further breakdown of the PPS fiber by oxidation degradation, the generation of oxides such as SO2, the breakdown of the phenol resin by these oxides, and further, the breakdown of the PPS fibers by the breakdown product of the phenol resin. Furthermore, even in the case of a meta-aramid fiber, para-aramid fiber, or polyimide fiber, in addition to the above-mentioned exhaust gas, oxides such as NOx are produced by oxidation degradation and hydrolysis, and these oxides are presumed to promote breakdown of the phenol resin. Furthermore, when a fluorofiber is used as well, although there is no generation of acidic gas such as SOx and NOx derived from the fiber, it is not possible to avoid the promotion of the breakdown of the phenol resin fiber due to acidic gas such as SOx and NOx in the exhaust gas.